Since the conclusion of the Republican and Democratic national conventions, pundits, political reporters and ordinary Americans have—for understandable reasons—been preoccupied with developments in the presidential campaign. With the presidential contest getting so much coverage in the national media, however, much less attention has been devoted to the critical battle for control of the next Congress. Regardless of the outcome of the presidential election, whether Republicans or Democrats control the House and Senate will have enormous consequences on the ability of the next president to carry out his agenda.

At present, Democrats hold a 232-198 seat majority in the House of ...